linux-kernel - Re: [PATCH v3 1/5] slab: Correct size_index table before replacing the bootstrap kmem_cache

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Date:	Thu, 05 Feb 2015 10:37:32 +0200
From:	Pekka Enberg <penberg@....fi>
To:	Daniel Sanders <daniel.sanders@...tec.com>
CC:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] slab: Correct size_index table before replacing
 the bootstrap kmem_cache_node.


On 02/04/2015 11:06 PM, Daniel Sanders wrote:
> This patch moves the initialization of the size_index table slightly
> earlier so that the first few kmem_cache_node's can be safely allocated
> when KMALLOC_MIN_SIZE is large.
>
> There are currently two ways to generate indices into kmalloc_caches
> (via kmalloc_index() and via the size_index table in slab_common.c)
> and on some arches (possibly only MIPS) they potentially disagree with
> each other until create_kmalloc_caches() has been called. It seems
> that the intention is that the size_index table is a fast equivalent
> to kmalloc_index() and that create_kmalloc_caches() patches the table
> to return the correct value for the cases where kmalloc_index()'s
> if-statements apply.
>
> The failing sequence was:
> * kmalloc_caches contains NULL elements
> * kmem_cache_init initialises the element that 'struct
>    kmem_cache_node' will be allocated to. For 32-bit Mips, this is a
>    56-byte struct and kmalloc_index returns KMALLOC_SHIFT_LOW (7).
> * init_list is called which calls kmalloc_node to allocate a 'struct
>    kmem_cache_node'.
> * kmalloc_slab selects the kmem_caches element using
>    size_index[size_index_elem(size)]. For MIPS, size is 56, and the
>    expression returns 6.
> * This element of kmalloc_caches is NULL and allocation fails.
> * If it had not already failed, it would have called
>    create_kmalloc_caches() at this point which would have changed
>    size_index[size_index_elem(size)] to 7.
>
> Signed-off-by: Daniel Sanders <daniel.sanders@...tec.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Pekka Enberg <penberg@...nel.org>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org

Acked-by: Pekka Enberg <penberg@...nel.org>
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